1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more specifically to catadioptric optical systems used for bright-field and dark-field optical inspection applications.
2. Description of the Related Art
Many optical and electronic systems exist to inspect surface features for defects such as those on a partially fabricated integrated circuit or a reticle. Defects may take the form of particles randomly localized on the surface, scratches, process variations such as under etching, and so forth. Such techniques and apparatus are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif.
Several different imaging modes exist for optical inspection. These include bright-field, and a variety of dark-field imaging modes. Each of these imaging modes can detect different types of defects. An ideal inspection machine would combine several different modes into a single system. This could reduce inspection costs as well as provide performance advantages. However, this is very difficult to do in practice because different imaging modes have different optical design, software, and system requirements. In general, systems that attempt to combine bright-field and dark-field imaging do not perform either mode as well as single-mode designs.
Bright-field imaging is commonly used in lamp based microscope systems. The advantage of bright-field imaging is the imaged features are typically readily distinguishable. Image feature size accurately represents the size of object features multiplied by the magnification of the optical system. Dark-field imaging is successfully used to detect features on objects. The advantage of dark-field imaging is that flat specular areas scatter very little light toward the detector, resulting in a dark image. Any surface anomalies or features protruding above the object scatter light toward the detector. Thus, in inspecting objects such as semiconductor wafers, dark-field imaging produces an image of features, particles, or other irregularities on a dark background. Normal incidence laser dark-field is a flexible imaging mode that is suited for detecting microscratches.
Examples of inspection systems include high numerical aperture (NA) catadioptric systems with central obscurations, such as those shown in U.S. Pat. No. 5,717,518 by Shafer et al., and U.S. Pat. No. 6,064,517 by Chuang et al. Bright field and dark field imaging is employed in, for example, U.S. Pat. No. 6,288,780 by Fairley et al., U.S. Pat. No. 5,153,668 by Katzir et al., and U.S. Pat. No. 5,058,982 by Katzir. Fourier filtering in laser dark field imaging has been employed in certain designs, such as those shown in U.S. Pat. No. 5,177,559 by Batchelder et al. and U.S. Pat. No. 5,428,442 by Lin et al.
Certain of the previous designs exhibit beam delivery issues when performing dark-field imaging. For example, high NA systems can have dark-field illumination capabilities limited by the central obscuration in the catadioptric optical design. In these systems dark-field illumination can be limited to angles greater than a certain value, such as five degrees. Off axis dark-field imaging and low NA bright-field imaging has been employed to address this angular illumination issue in catadioptric designs. In such a design, the dark-field illumination may be a high NA annular illumination scheme where bright-field illumination and imaging may be limited to those NAs that can pass through a hole in the center of the focusing mirror element. Such a construction can also limit dark-field illumination and collection angles. Another prior apparatus for combining bright-field and dark-field into one design uses off axis dark-field and low NA bright-field imaging. The dark-field illumination is a high NA linear type of illumination. The bright-field illumination and imaging is again limited to those NAs that can pass through a slot in the center of the focusing mirror elements, again limiting dark-field illumination and collection angles.
One prior method for achieving Fourier filtering with laser dark-field imaging uses a collimated beam of monochromatic light to illuminate a semiconductor wafer from outside the objective between an angle of 82 degrees from the normal and the NA defined by the imaging objective. Before forming a dark field image, the collected light passes through a Fourier filter to attenuate the spatial frequency components corresponding to repeating array patterns. This laser directional dark-field method illuminates the wafer outside the NA of the imaging objective. For this reason, the illumination angles can be limited to between 82 degrees from the normal and the NA defined by the imaging objective. Collection angles are also limited to the range of angles within the NA of the objective. A long working distance objective is necessary to allow access by the laser to the area of interest on the semiconductor wafer. Objectives used in dark field applications of this type are generally limited to NAs less than 0.7, which corresponding to collection angles of only up to 44 degrees from normal. A major drawback of this approach is the Fourier distribution collected by the optics is highly directional, where only higher angles are collected from one side of the scattered and diffracted light distribution.
Another prior method for achieving Fourier filtering with laser dark-field imaging uses a collimated beam of monochromatic light illuminating the wafer from inside the optical system within the NA defined by the objective. If the system encounters a specific range of defect sizes, the illumination angle on the wafer is chosen so the optical system collects those spatial frequencies of interest. This is a laser directional dark-field method wherein the laser illuminates the wafer from inside the NA as defined by the objective. The problem with this technique is that small amounts of scattered and reflected light from lens elements in this design have the ability to produce noise at levels that compromise sensitivity. Introducing laser illumination near the pupil location in the imaging system can cause a significant amount of back-scattered and reflected light from the multiple lens surfaces traversed by the illuminating light. The system must also deal with forward-scattered light from the specularly reflected component from the wafer, a significant potential problem. Another problem with this technique is that the system uses the same objective pupil plane for injecting the illumination and processing the light collected by the objective. This objective pupil feature limits the usable types of illumination and Fourier filtering. Achieving a higher NA is also difficult using this design. Higher NA lenses generally require more optical elements, further increasing scattering noise.
In general, it can be difficult to offer both bright-field and dark-field imaging in a high NA inspection system while offering Fourier filtering, operation over a desirable range of wavelengths, and dark-field illumination over a desirable range of angles.
It would therefore be beneficial to provide a system that delivers a beam of laser dark-field illumination that overcomes the foregoing drawbacks present in previously known imaging systems. Further, it would be beneficial to provide an optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.